Visible-Light-Promoted Cascade Radical Cyclization: Synthesis of 1,4-Diketones Containing Chroman-4-One Skeletons

Article abstract of DOI:10.1002/asia.201901078

A visible-light-induced cascade radical cyclization of aroyl chlorides with 2-(allyloxy)-benzaldehyde derivatives has been developed. The method takes advantages of unactivated C=C bonds as the acyl radical acceptors and offers a mild and green approach for the synthesis of 1,4-diketones bearing biologically important chroman-4-one skeletons with moderate to good yields.

Full text of DOI:10.1002/asia.201901078

DOI: 10.1002/asia.201901078
Communication
Visible-Light-Promoted Cascade Radical Cyclization: Synthesis of
1,4-Diketones Containing Chroman-4-One Skeletons
Xiang-Kui He,[a] Bao-Gui Cai,[a] Qing-Qing Yang,[b] Long Wang,*[b] and Jun Xuan*[a, c]
In the past decade, chemical reactions initiated by visible
light-induced photoredox catalysis has attracted considerable
attentions because of its environmental compatibility and ver-
satility in character.[5] A wide range of useful chemical struc-
tures can be easily accessed by using this benign methodolo-
gy, including the synthesis of 1,4-diketones. Generally, visible-
light promoted synthesis of 1,4-diketone derivatives can be di-
vided into the following two categories. The first approach is
the addition of photo-generated carbon-centered radical spe-
cies to unsaturated carbon-carbon double bonds (Scheme 1,
method A). In 2014, the group of Xiao reported a visible-light
induced 1,4-diketone synthesis by using b-ketosulfones as the
carbon-centered radical precursors.[6a] Very recently, Zhu et al.
disclosed an elegant photoredox catalytic approach to fluoro-
containing 1,4-diketones by using ethyl bromodifluoroacetate
as the fluoroalkyl radical source.[6b] Another visible light photo-
catalytic method to the synthesis of 1,4-diketones are based
on the conjugate addition of acyl radical to olefins (Scheme 1,
method B).[7] In 2015, Shang, Fu et al. demonstrated a visible
Abstract: A visible-light-induced cascade radical cycliza-
tion of aroyl chlorides with 2-(allyloxy)-benzaldehyde de-
rivatives has been developed. The method takes advan-
tages of unactivated C=C bonds as the acyl radical accept-
ors and offers a mild and green approach for the synthesis
of 1,4-diketones bearing biologically important chroman-
4-one skeletons with moderate to good yields.
The 1,4-diketones are versatile synthetic building blocks which
have been wide used in the synthesis of important carbo- and
heterocyclic ring structures, such as cyclopentenones, pyrroles,
furans and thiophenes.[1] As the structural elements, they can
be found in a wide range of pharmaceutical agents and natural
isolates.[2] Consequently, there is no doubt that significant ef-
forts have been devoted to the synthesis of this highly valua-
ble synthon. Among the reactions developed, Michael addi-
tion-types of acyl radical to a,b-unsaturated carbonyl com-
pounds is one of the most straightforward and powerful syn-
thetic approach to access 1,4-diketones.[3] However, traditional
methods for the generation of acyl radicals generally need the
use of toxic organotin initiators, UV irradiation, high reaction
temperature, or high CO pressure.[4] Thus, further explore of
valuable process for the generation of acyl radicals and their
application in the synthesis of important 1,4-diketones is still
appealing, especially under green and mild reaction condi-
tions.
[a] X.-K. He, B.-G. Cai, Prof. Dr. J. Xuan
Anhui Province Key Laboratory of Chemistry for Inorganic/Organic
Hybrid Functionalized Materials
College of Chemistry & Chemical Engineering
Anhui University
Hefei, Anhui 230601 (P. R. China)
E-mail: xuanjun@ahu.edu.cn
[b] Dr. Q.-Q. Yang, Prof. Dr. L. Wang
China Three Gorges University
College of Materials and Chemical Engineering
Key laboratory of inorganic nonmetallic crystalline and energy conversion
materials
Yichang, Hubei 443002 (P. R. China)
E-mail: wanglongchem@ctgu.edu.cn
[c] Prof. Dr. J. Xuan
Key Laboratory of Structure and Functional Regulation of Hybrid Materials
(Anhui University)
Ministry of Education
Hefei 230601 (P. R. China)
Supporting information and the ORCID identification number(s) for the au-
thor(s) of this article can be found under:
https://doi.org/10.1002/asia.201901078.
Scheme 1. Visible Light-promoted synthesis of 1,4-diketones.
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light photoredox-catalyzed decarboxylative 1,4-addition reac-
tion, in which a-oxocarboxylic acids were employed to gener-
ate acyl radicals.[8a] Shortly after this elegant discovery, other
types of acyl radical precursors, including carboxylic anhy-
drides,[9] aldehydes,[10] 2-S-pyridyl thioesters,[11] aroyl chlor-
ides,[12] 4-acyl-1,4-dihydropyridines[13] and oxime esters[14] were
sequentially investigated to conduct such valuable radical ad-
dition reactions. Although encouraging improvements have
been achieved in this field, the above photo-generated acyl
radicals are all limited addition to electron-deficient alkenes. It
is still great appealing to explore the potential reactivity of
acyl radicals, especially in the reaction with unactivated C=C
bonds and the construction of molecular complexity under
visible light photoredox catalytic conditions.
temperature (entry 1). Solvent screening revealed that DMF
also gave a comparable yield (entry 2, 57%), while other tested
reaction medias such as DCM, DCE, EtOAc, THF, and 1,4-diox-
ane were all inferior in terms of reaction yields (entries 3–7). It
was found that the base had a significant effect on the reac-
tion efficiency. Only 35% yield of 3a was obtained when 2,6-
lutidine was replaced by Et3N (entry 8). And no desired annula-
tion product was observed by using inorganic base, such as
K2CO3 and Cs2CO3 (entries 9 and 10). To our delight, the reac-
tion yield of 3a could be further improved to 65% when the
ratio of 1a to 2a was improved from 2:1 to 1:2 (entry 11). Fi-
nally, control experiments indicated that the photoredox cata-
lyst, blue LED irradiation, and 2,6-lutidine are all crucial to this
cascade radical cyclization process (entries 12–14).
Within the frame of our ongoing research interests devoted
to the synthesis of valuable heterocyclic compounds,[15] we de-
scribe herein a visible light-promoted cascade radical cycliza-
tion of aroyl chlorides with o-(allyloxy)arylaldehydes
(Scheme 1). It is worth noting that the photo-generated acyl
radical can be efficiently trapped by un-activated C=C bond in
this process. The reaction proceeds through a radical cycliza-
tion/1,2-H migration/oxidative ketone formation cascade which
enables the one-step, efficient synthesis of various 1,4-dike-
tones bearing biologically important chroman-4-one skele-
tons.[16]
With the optimal reaction conditions in hand, we next set
out to investigate the substrate scope for the reaction
(Scheme 2). It was found that this photocatalytic cascade radi-
cal cyclization reaction can tolerate a wide range of substitu-
ents on the benzene ring of 2-(allyloxy)-benzaldehyde deriva-
tives. Both electron-donating (e.g., methyl, methoxy) and elec-
tron-withdrawing (e.g., fluoro, chloro, bromo) groups could be
successfully introduced at different positions on the aryl ring
of 2, affording the corresponding chroman-4-ones 3b–g in
moderate to good yields. Moreover, the disubstituted and
naphthalene derived substrates worked well to provide 3h
and 3i in 40% and 42% isolated yields, respectively.
Next, the scope of the reaction with respect to aryl aroyl
chlorides 1 was examined (Scheme 2). It was observed that
electronic modification of aroyl chlorides 1 had some effects
on the reaction efficiency. Aroyl chlorides bearing electron-do-
nating (-Me, -OMe) and weak electron-withdrawing (-F, -Cl, -Br)
groups could smoothly react with 2-(allyloxy)-benzaldehyde 2a
under the optimized conditions, giving the resulting 1,4-dike-
tones 3j–m, 3q in generally good yields. However, 4-nitroben-
zoyl chloride and 4-cyanobenzoyl chloride did not react at all
which might because of the strong electron-withdrawing
group reduce the addition susceptibility of acyl radical to C=C
bonds (3n and 3o).[16b] In addition, a relatively low yield of the
desired product was obtained when 2-bromobenzoyl chloride
was involved which could be attributed to steric effects (3p,
30%). Apart from benzoyl chloride derivatives, thiophene-2-
carbonyl chloride also proved to be a reliable substrate, afford-
ing 3r in 61% yield. It is worth noting that heterocycles 3s-w
bearing an important quaternary carbon center, could be un-
ambiguously produced by using this developed method.
At the outset, benzoyl chloride 1a and 2-(allyloxy)-benzalde-
hyde 2a was selected as model substrates to identify the reac-
tion efficiency. As revealed in Table 1, the desired annulation
product 3a could be isolated in 61% yield when the reaction
was performed in the presence of 2.0 mol% fac-Ir(ppy)3 as
photoredox catalyst, 2.0 equiv 2,6-lutidine as base, in degassed
CH3CN under irradiation with 24 W blue LED for 24 h at room
Table 1. Reaction optimization between 1a and 2a.[a]
Entry
Deviation from standard conditions
Yield [%][b]
1
None
61
2
3
4
5
6
7
8
9
10
11
12
13
14
DMF instead of CH3CN
DCM instead of CH3CN
DCE instead of CH3CN
EA instead of CH3CN
THF instead of CH3CN
1,4-dioxane instead of CH3CN
Et3N instead of 2,6-lutidine
K2CO3 instead of 2,6-lutidine
Cs2CO3 instead of 2,6-lutidine
1a:2a=1:2
57
52
45
48
43
51
35
trace
trace
65
n.r.
n.r.
n.r.
To demonstrate the preparative utility of this method, the
follow-up chemistry was investigated as shown in Scheme 3.
First, Moderate yield of 3u was obtained when the reaction
was performed by direct sunlight irradiation (Scheme 3a).
More significantly, the chromanone product 3a could be effi-
ciently transfer to benzofuranone 4 in 75% yield through a
base-catalyzed rearrangement process (Scheme 3b).[17a] It is
well known that 1,4-diketones are indispensable building
blocks for the synthesis of structurally specific heterocyclic ring
structures.[1] For instance, treatment of 3a with Me3SiCl in
MeOH at 908C provided polyheterocycle 5 in good yield (Sche-
me 3c).[17b] The 2-phenyl-4H-thieno[3,2-c]chromene 6 could
Without fac-Ir(ppy)3
In the dark
Without 2,6-lutidine
[a] Reaction conditions: 1a (0.4 mmol), 2a (0.2 mmol), 2,6-lutidine
(0.4 mmol), fac-Ir(ppy)3 (0.004 mmol) in the indicated solvent (2.0 mL), ir-
radiation by 24 W blue LEDs at room temperature for 24 h under argon
atmosphere. [b] Isolated yield.
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Scheme 2. Reaction scopes.[a,b] [a] 1 (0.1 mmol), 2 (0.2 mmol), 2,6-lutidine (0.2 mmol), fac-Ir(ppy)3 (0.002 mmol) in CH3CN (1.0 mL), irradiation by 24w blue LEDs
at room temperature for 24 h under argon atmosphere. [b] Isolated yield.
Volmer fluorescence quenching experiments by using benzoyl
chloride and 2,6-lutidine as the quencher indicated that benzo-
yl chloride could efficiently quench the photo-excited fac-
Ir(ppy)3, where it presumably engaged in a SET process
(Scheme 4a). Moreover, the model reaction was completed in-
hibited when 2.0 equiv TEMPO was added to the reaction mix-
ture (Scheme 4b). Instead, the radical trapping product 7 was
isolated in 50% yield, indicating the formation of benzoyl radi-
cal.
Based on the experimental evidence in combination with
previous literature reports, a plausible reaction mechanism was
proposed in Scheme 5. Initially, fac-Ir(ppy)3 transferred to its
photo-excited state under the irradiation with visible light.
Then, single-electron reduction of benzoyl chloride with IrIII*
provided the key benzoyl radical A with the formation of IrIV
species.[18] Radical addition of A to C=C bond of 2-(allyloxy)-
benzaldehyde afforded radical B, which subsequently under-
Scheme 3. Sun-light irradiation reaction and synthetic transformations of 3a.
went an intramolecular radical addition to give oxygen-cen-
tered radical C. 1,2-hydrogen migration of C delivered radical
also be easily prepared in high yield by the reaction of 3a with
Lawesson’s reagent (Scheme 3d).[6]
D,[19] which could serve as reductant to reduce IrIV to IrIII to
complete the photocatalytic cycle and yield the carbon cation
intermediate E. Finally, base-promoted deprotonation of E pro-
Some control experiments were subsequently conducted to
deepen our understanding of the reaction mechanism. Stern– vided the final chroman-4-one product.
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Scheme 4. Mechanistic studies.
tor. The crude residue was purified by silica gel column chromatog-
raphy to afford pure product 3a as a yellow solid in 65% yield.
Acknowledgements
We are grateful to the National Natural Science Foundation of
China (No. 21702001, No. 21602123), and the Start-up Grant
from Anhui University for financial support of this work.
Conflict of interest
Scheme 5. Proposed reaction mechanism.
The authors declare no conflict of interest.
Keywords: 1,4-Diketones · Acyl radical · Photoredox catalysis ·
Radical cyclization · Visible light
In summary, we have developed a visible-light-promoted
cascade radical cyclization of aroyl chlorides with 2-(allyloxy)-
benzaldehyde derivatives. The reaction proceeded through a
radical cyclization/1,2-H migration/oxidative ketone formation
cascade which provided an efficient route to various 1,4-dike-
tones bearing biologically important chroman-4-one skeletons
in moderate to good yield. Comparing with the well-devel-
oped activated alkenes as radical acceptors, the photo-gener-
ated acyl radical was efficiently trapped by the unactivated C=
C bond in this process. Moreover, the sun-light irradiation reac-
tion and the synthetic transformation of the formed 1,4-dike-
tones to other important polyheterocycles further rendered
this approach attractive and valuable.
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Experimental Section
General procedure: A flame-dried Schlenk-tube equipped with a
magnetic stir bar was charged with 1a (1.0 equiv, 0.1 mmol), fac-
Ir(ppy)3 (0.02 equiv, 0.002 mmol) in 1.0 mL CH3CN was added 2a
(2.0 equiv, 0.2 mmol) and 2,6-lutidine (2.0 equiv, 0.2 mmol) under a
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Manuscript received: August 6, 2019
Revised manuscript received: August 28, 2019
Accepted manuscript online: August 29, 2019
Version of record online: September 10, 2019
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